After the completion of the 30 minutes assessment period, each patient will receive a sciatic nerve block and spinal anesthesia. Sciatic nerve block will be performed using the subgluteal approach with 20ml of 0.5% plain ropivicaine. Spinal anesthesia will be performed in the lateral position with the operated side uppermost. Intrathecal isobaric bupivacaine 10 mg with 10 µg of fentanyl will be injected under sterile conditions using 25 G whitacre needle.